Managing distributed access to a shared medium

ABSTRACT

A method includes determining that a first station has been allocated a first time period to transmit over a shared medium in a network. The method includes transmitting, from the first station to a second station over the shared medium during the first time period, wherein stations other than the first station and the second station refrain from transmitting over the shared medium during the first time period. The method includes receiving, from the second station, a request message to allow the second station to transmit during the first time period and a requested amount of time to transmit. The method includes, in response to allowing the second station to transmit during the first time period, determining an authorized amount of time for the second station to transmit during the first time period, and transmitting an authorization message for the second station to transmit and the authorized amount of time.

RELATED APPLICATIONS

This application claim priority benefit of and is a continuation of U.S.application Ser. No. 12/118,613, filed on May 9, 2008, which claimspriority benefit of U.S. Provisional Application No. 60/917,232, filedMay 10, 2007.

TECHNICAL FIELD

The invention relates to network protocols and more particularly tomanaging distributed access to a shared medium.

BACKGROUND

Communication stations can share a communication medium using any of avariety of access techniques. Some access techniques (e.g., carriersense multiple access (CSMA) techniques) include a contention period inwhich stations determine contend for use of the medium for transmittinga signal by sensing when the medium is idle. In CSMA techniques,“collisions” sometimes occur when signals from two or more stationsoverlap. Some CSMA techniques attempt to detect collisions and aborttransmission to reduce the negative impact of collisions (e.g., CSMA/CDtechniques). Other CSMA techniques include mechanisms to avoid or reducethe probability of collisions (e.g., CSMA/CA techniques). For example,different transmissions may be assigned one of multiple priorities.Access is granted using a Priority Resolution Period in which stationssignal the priority at which they intend to transmit, and only thehighest priority transmissions are allowed to continue in the contentionprocess. A random backoff mechanism spreads the time over which stationsattempt to transmit, thereby reducing the probability of collision.

Other access techniques (e.g., time division multiplexing (TDM)techniques) allocate predetermined time intervals in which certainstations are granted use of the medium. A particular station willtransmit within a time slot assigned to that station. In thesetechniques, a synchronization mechanism is used to ensure that thestations agree about when their slot occurs with respect to a commontime reference.

SUMMARY

Some embodiments include a method for communicating between stations ina network. The method includes determining that a first station has beenallocated a first time period to transmit over a shared medium in thenetwork. The method includes transmitting, from the first station to asecond station over the shared medium during the first time period,wherein stations other than the first station and the second stationrefrain from transmitting over the shared medium during the first timeperiod. The method includes receiving, from the second station by thefirst station, a request message to allow the second station to transmitduring the first time period and a requested amount of time to transmit.The method includes determining, by the first station, whether to allowthe second station to transmit during the first time period. The methodincludes, in response to allowing the second station to transmit duringthe first time period, determining, by the first station, an authorizedamount of time for the second station to transmit during the first timeperiod, and transmitting, by the first station to the second station, anauthorization message for the second station to transmit and theauthorized amount of time.

Some embodiments include a method for communicating between stations ina network. The method includes determining that a first station has beenallocated a first time period to transmit over a shared medium in thenetwork based on a contention period to determine selection among thestations. The method includes transmitting, from the first station to asecond station over the shared medium during the first time period,wherein stations other than the first station and the second stationrefrain from transmitting over the shared medium during the first timeperiod. In response to receiving, by the first station, a requestmessage from the second station that includes a requested amount of timethat is requested for transmission by the second station, the firststation transmits an authorization message to the second station totransmit for an authorized amount of time during the first time period,and transmits the authorized amount of time to the second station.

In one aspect, in general, a method for communicating between stationsin a network is described. The method includes coordinating among aplurality of the stations according to a distributed protocol to selecta first station to transmit over a shared medium. The method alsoincludes transmitting between the first station and a second stationover the shared medium during a time period in which stations other thanthe first and second stations refrain from transmitting over the sharedmedium. The first station transmits information that grants permissionto the second station to transmit during the time period.

Aspects may incorporate one or more of the following features.

Transmitting between the first station and the second station furtherincludes transmitting from the first station information that includes aspecified amount of the time period during which the second station isallowed to transmit.

A specified amount of the time period during which the second station isallowed to transmit is predetermined before the time period begins.

The specified amount of the time period is stored in each of thestations.

The specified amount of the time period is stored in a given stationwhen the given station joins the network.

The specified amount of time is determined as a function of a data ratebeing used by a station.

The method further comprises coordinating among the plurality ofstations according to the distributed protocol to select one of thestations to transmit over the shared medium after receiving informationover the shared medium from the first station or the second station thatindicates that the time period has ended.

The first station or the second station transmits the information thatindicates that the time period has ended in response to receiving atransmission from the second station or the first station.

The information that indicates that the time period has ended includesacknowledgement information confirming successful transmission ofinformation from the second station or the first station.

The first station or the second station transmits the information thatindicates that the time period has ended in response to detecting apotential collision.

Detecting a potential collision comprises receiving a transmission inwhich at least some data is not correctly received.

Detecting a potential collision comprises not receiving any transmissionover the shared medium after a predetermined amount of time.

The first station transmits the information that grants permission afterreceiving a request from the second station.

The request from the second station is transmitted to the first stationafter at least one transmission from the first station to the secondstation during the time period.

The method further comprises providing a plurality of opportunities forthe second station to transmit to the first station during at least aportion of the time period.

The time period includes time during which the second station is able totransmit acknowledgement information confirming successful transmissionof information from the first station to the second station.

The specified amount of the time period during which the second stationhas received permission to transmit is larger than the time during whichthe second station transmits the acknowledgement information.

The first station transmits the information that grants permission afterreceiving a request from the second station included in theacknowledgement information.

The acknowledgement information includes information designating whichof multiple segments of information were successfully received by thesecond station and which of the multiple segments of information shouldbe retransmitted by the first station.

The method further comprises transmitting from the second station duringthe time period in response to receiving the information that grantspermission.

The method further comprises transmitting from the first stationacknowledgement information confirming successful transmission ofinformation from the second station to the first station.

The acknowledgement information includes information designating whichof multiple segments of information were successfully received by thefirst station and which of the multiple segments of information shouldbe retransmitted by the second station.

Information transmitted from the second station within the specifiedamount of the time period includes information other thanacknowledgement information confirming successful transmission ofinformation from the first station to the second station.

The information transmitted from the second station within the specifiedamount of the time period includes a delimiter that precedes a payload,wherein the payload includes the information other than acknowledgementinformation.

The delimiter includes acknowledgement information confirming successfultransmission of information previously transmitted from the firststation to the second station.

The information that grants permission is included in header informationof a transmission from the first station to the second station.

The first station transmits the information that grants permission afterreceiving a transmission from the second station that includes an amountof time that is requested.

Information transmitted from the second station to the first stationwithin the specified amount of the time period comprises informationused to maintain data flow from the first station to the second station.

The information transmitted from the second station to the first stationwithin the specified amount of the time period comprises adaptationinformation used by the first station to prepare a signal to betransmitted to the second station.

The adaptation information comprises a map that designates a type ofmodulation that is to be used, respectively, on each of multiplecarriers in the signal.

Information transmitted from the second station to the first stationwithin the specified amount of the time period comprises informationaccording to a protocol associated with a connection between the firststation and the second station.

The protocol comprises transmission control protocol (TCP).

The protocol comprises a voice over internet protocol (VoIP).

Coordinating among the plurality of stations according to thedistributed protocol comprises contending according to a carrier sensemultiple access (CSMA) protocol.

Coordinating among the plurality of stations according to thedistributed protocol comprises selecting a predetermined sequence oftime slots to which respective stations are assigned.

The predetermined sequence of time slots repeats periodically.

In another aspect, in general, a system for communicating betweenstations is described. The system includes a first station configured tocoordinate among a plurality of the stations according to a distributedprotocol to gain access to transmit over a shared medium, and transmitto a second station over the shared medium during a time period in whichstations other than the first and second stations refrain fromtransmitting over the shared medium, including transmitting informationthat grants permission to the second station to transmit during the timeperiod. The second station is configured to, in response to receivingthe information that grants permission, transmit to the first stationover the shared medium during the time period.

In another aspect, in general, a method for communicating betweenstations in a network is described. The method includes coordinatingamong a plurality of the stations according to a distributed protocol toselect a first station to transmit over a shared medium; transmittingbetween the first station and a second station over the shared mediumduring a time period in which stations other than the first and secondstations refrain from transmitting over the shared medium; andcoordinating among the plurality of stations according to thedistributed protocol to select one of the stations to transmit over theshared medium after receiving information over the shared medium fromthe first station or the second station that indicates that the timeperiod has ended.

Aspects can include one or more of the following features.

The method further comprises transmitting from the first stationinformation that grants permission to the second station to transmitduring the time period.

The first station or the second station transmits the information thatindicates that the time period has ended in response to receiving atransmission from the second station or the first station.

The information that indicates that the time period has ended includesacknowledgement information confirming successful transmission ofinformation from the second station or the first station.

The first station or the second station transmits the information thatindicates that the time period has ended in response to detecting apotential collision.

Detecting a potential collision comprises receiving a transmission inwhich at least some data is not correctly received.

Detecting a potential collision comprises not receiving any transmissionover the shared medium after a predetermined amount of time.

In another aspect, in general, a system for communicating betweenstations is described. The system includes a first station configured tocoordinate among a plurality of the stations according to a distributedprotocol to gain access to transmit over a shared medium, and transmitto a second station over the shared medium during a time period in whichstations other than the first and second stations refrain fromtransmitting over the shared medium. The second station is configured totransmit to the first station over the shared medium during the timeperiod. A third station is configured to coordinate among the pluralityof stations according to the distributed protocol to gain access totransmit over the shared medium after receiving information over theshared medium from the first station or the second station thatindicates that the time period has ended.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

It enables stations to operate reliably and at higher data rates undervarious power line environments. It provides a channel adaptationmechanism that can be used in power line communication systems as wellas other media that are affected by channel impairments. It can providea higher level of guaranteed quality of service (QoS). It enables moreefficient utilization of contention-free time allocations by enabling atransmitting station to grant some of its allocated time to a receivingstation.

The time granted from the allocated time enables a receiving station tosend information other than acknowledgement information. For example,time allocated to a first station may include time during which a secondreceiving station is able to transmit acknowledgement information to thefirst station confirming successful transmission of information from thefirst station to the second station. The bi-directional burstingprocedure preserves the ability of the receiving station to transmitsuch acknowledgment information without obtaining specific permission,and provides a way for the receiving station to obtain permission totransmit further information.

At the end of a burst, stations that did not participate in the burstcan detect an indicator within a final transmission that signals thatthe burst is ending and that the stations are able to start contendingfor access to the medium. This indicator enables the stations to startcontending quickly without needing to wait for an inter-frame spacing.This reduces the delay and also avoids the need for sensing circuitryfor detecting the lack of a signal during such an inter-frame spacing.

Other features and advantages of the invention will be found in thedetailed description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a network configuration.

FIG. 2 is a block diagram of a communication system.

FIG. 3 is a timing diagram of an example of bursting.

FIGS. 4, 5, 6A, and 6B are timing diagrams of examples of bidirectionalbursting.

DESCRIPTION OF EMBODIMENT(S)

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

System Overview

As shown in FIG. 1, a network configuration 100 provides a sharedcommunication medium 110 for a number of communication stations (e.g.,computing devices, or audiovisual devices) to communicate with eachother. The communication medium 110 can include one or more types ofphysical communication media such as coaxial cable, unshielded twistedpair, or power lines, for example. The network configuration 100 canalso include devices such as bridges or repeaters. The communicationstations communicate with each other using predetermined physical (PHY)layer and medium access control (MAC) layer communication protocols. TheMAC layer is a sub-layer of the data link layer and provides aninterface to the PHY layer, according to the Open SystemsInterconnection (OSI) network architecture standard. The networkconfiguration 100 can have any of a variety of network topologies (e.g.,bus, tree, star, mesh). The communication stations communicate with oneanother based on requests from software applications running on thehardware of the respective station.

The stations can have differences in the specific communicationprotocols used, and are still able to communicate with each other if theprotocols are compatible. For example, network configuration 100includes a first type of communication station including stations 102A,102B, 102C that use a first MAC layer protocol “MAC-A” with a secondtype of communication station including stations 104A and 104B that usea second type of MAC layer protocol “MAC-B.” The MAC-A and MAC-Bprotocols are compatible with each other and also use the same or atleast compatible PHY layer protocols (e.g., one station uses a MAC-Aprotocol and a PHY-A protocol, and another station uses a MAC-B protocoland a PHY-B protocol, where PHY-A and PHY-B implement compatible signalmodulation formats).

The co-existence of multiple MAC layer protocols can be used, forexample, to allow improvements in capabilities and/or performance of theMAC layer while also allowing devices using a new MAC layer protocol tobe compatible with legacy devices using an older MAC layer protocol thatmay exist in the network configuration 100. In some implementations, adual-mode (DM) device can communicate with a legacy single-mode (SM)device using a first protocol, and can communicate with other DM devicesusing either the first protocol or a second protocol. The protocol to beused can be set by a communication mode that is determined at networksetup time or when a device joins the network. For example, stations104A and 104B include a network interface module 108 that uses MAC-A.Stations 102A, 102B, and 102C include a network interface module 106that can use either MAC-A or MAC-B depending on the determinedcommunication mode.

An example of a difference between the protocol layers (or “protocolstack”) used by different types of devices (e.g., the DM and SM devices)is the use of a “central coordinator” (CCo) station. The CCo is acommunication station that is selected to provide certain coordinationfunctions for at least some of the other stations in the networkconfiguration 100. A set of stations operating under the coordination ofa single CCo is called a Basic Service Set (BSS). Functions performed bythe CCo include: authentication of stations upon joining the BSS,provisioning of identifiers for stations, and scheduling and timing ofmedium access. For example, the CCo broadcasts a repeated beacontransmission from which the stations in the BSS can determine schedulingand timing information. This beacon transmission includes fields thatcarry information used by the stations to coordinate communication.Though the format of each of the repeated beacon transmission issimilar, the content typically changes in each transmission. The beacontransmission is repeated approximately periodically, and, in someimplementations, is synchronized to a characteristic of thecommunication medium 110. The time between successive beacontransmissions is called a “beacon period,” even though the transmissionsmay not be exactly periodic. In some cases, a Proxy Coordinator (PCo)can be used to manage stations that are “hidden” from the CCo (e.g.,stations that do not reliably receive signals from the CCo). An exampleof a system in which a CCo is used is described in U.S. application Ser.No. 11/337,963, incorporated herein by reference.

Some protocols are distributed protocols that do not rely on a CCostation to coordinate communication. Instead, the stations coordinateamong each other according to a distributed protocol in which one of thestations is selected to transmit over a shared medium during a giventime period. An example of such a distributed protocol is Carrier SenseMultiple Access with Collision Avoidance (CSMA/CA). In this distributedprotocol, stations use random backoff algorithms to determine how long astation has to wait before it transmits a packet. If the stationdetermines that another station has already started a transmission, thestation will wait until the current transmission is complete, afterwhich it uses the backoff algorithm. In general, the backoff parameterscan be adjusted in a distributed manner based on the number of times thestation deferred or the number of time a station's transmission hascollided with transmissions from one or more other stations. Thisdistributed adaptation of back-off parameters can enable the network towork efficiently under various network loads (or varying number oftransmitters). Distributed protocols can also be used to reserverepeating TDMA allocations on the medium. One such example is the onedescribed in U.S. application Ser. No. 10/695,371, entitled “ContentionFree Access Intervals on a CSMA network,” incorporated herein byreference. A principle behind such protocols is to notify neighboringstations about the repeating time intervals during which the stationintends to transmit. Once this information is successfully communicatedto the neighboring stations, the transmitter will be allocated therepeating TDMA time slot (or “allocation”) during which it will be freeto transmit. All stations in the network can use this procedure toobtain their own TDMA allocations based on application requirements.

There may also be differences in the access techniques implemented bythe MAC-A and MAC-B protocols. For example, in one scenario, the MAC-Aprotocol uses a first access technique the MAC-B protocol is compatiblewith the first access technique and provides a second access technique.In this example, the MAC-A protocol uses a CSMA/CA technique to accessthe network configuration 100. The MAC-B protocol uses a hybrid approachthat includes a contention-free period (CFP) in which a time divisionmultiple access (TDMA) technique is used, and optionally includes acontention period (CP) in which a CSMA/CA technique is used. Thecontention-free period is scheduled and managed by the CCo to provideimproved quality of service (QoS) for certain applications run on adevice (e.g., audio and/or video applications). Other MAC protocols canuse any one or combination of these or other access techniques.

In some implementations, the network interface modules use protocolsthat include features to improve performance when the networkconfiguration 100 includes a communication medium 110 that exhibitsvarying transmission characteristics. For example, one aspect ofmitigating potential impairments caused by the varying channelcharacteristics involves using a robust signal modulation format such asorthogonal frequency division multiplexing (OFDM), also known asDiscrete Multi Tone (DMT). OFDM is a spread spectrum signal modulationtechnique in which the available bandwidth is subdivided into a numberof narrowband, low data rate channels or “carriers.” To obtain highspectral efficiency, the spectra of the carriers are overlapping andorthogonal to each other. Data are transmitted in the form of symbolsthat have a predetermined duration and encompass some number ofcarriers. The data transmitted on these carriers can be modulated inamplitude and/or phase, using modulation schemes such as Binary PhaseShift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit QuadratureAmplitude Modulation (m-QAM).

System Architecture

Any of a variety of communication system architectures can be used toimplement the portion of the network interface module that converts datato and from a signal waveform that is transmitted over the communicationmedium. An application running on a station provides and receives datato and from the network interface module in segments. A “MAC ProtocolData Unit” (MPDU) is a segment of information including overhead andpayload fields that the MAC layer has asked the PHY layer to transport.An MPDU can have any of a variety of formats based on the type of databeing transmitted. A “PHY Protocol Data Unit (PPDU)” refers to themodulated signal waveform representing an MPDU that is transmitted overthe power line.

The MPDU can be formatted, for example, as a “frame” that includes astart of frame (SOF) delimiter and (optionally) a payload. In someimplementations an end of frame (EOF) delimiter can be used to indicatethe end of a frame, but an EOF delimiter is not necessary and is notused in the exemplary implementations illustrated in the figures. TheSOF delimiter can include overhead information within fields of a blockcalled “frame control” that is useful for decoding the payload or forresponding to another station according to a given protocol, forexample. The payload can be various lengths by including some number ofblocks to be individually encoded/modulated called PHY Blocks (PBs). Insome cases, no payload needs to be sent, but a small amount ofinformation such as acknowledgement information can be sent in a “shortMPDU” consisting of just an SOF delimiter without a payload. At thefront of a PPDU is a preamble that is used to synchronize a station tothe timing of the PPDU. In some cases, the preamble can be consideredpart of the MPDU, for example, with the SOF delimiter including apreamble and a frame control block.

In OFDM modulation, data are transmitted in the form of OFDM “symbols.”Each symbol has a predetermined time duration or symbol time T_(s). Eachsymbol is generated from a superposition of N sinusoidal carrierwaveforms that are orthogonal to each other and form the OFDM carriers.Each carrier has a peak frequency f_(i) and a phase Φ_(i), measured fromthe beginning of the symbol. For each of these mutually orthogonalcarriers, a whole number of periods of the sinusoidal waveform iscontained within the symbol time T_(s). Equivalently, each carrierfrequency is an integral multiple of a frequency interval Δf=1/T_(s).The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can beindependently selected (according to an appropriate modulation scheme)without affecting the orthogonality of the resulting modulatedwaveforms. The carriers occupy a frequency range between frequencies f₁and f_(N) referred to as the OFDM bandwidth.

There can be different types of PPDU structures, for example, dependingon whether the PHY-A or PHY-B protocol is being used. For example, thePHY-B signals can use denser OFDM carrier frequency spacing andcorrespondingly longer symbols.

Referring to FIG. 2, a communication system 200 includes a transmitter202 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 204 to a receiver 206. The transmitter 202 andreceiver 206 can both be incorporated into a network interface module ateach station. The communication medium 204 can represent a path from onedevice to another over the power line network.

At the transmitter 202, modules implementing the PHY layer receive anMPDU from the MAC layer. The MPDU is sent to an encoder module 220 toperform processing such as scrambling, error correction coding andinterleaving.

The encoded data is fed into a mapping module 222 that takes groups ofdata bits (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12 bits),depending on the constellations used for the various carriers (e.g., aBPSK, QPSK, 8-QAM, 16-QAM constellation), and maps the data valuerepresented by those bits onto the corresponding amplitudes of in-phase(I) and quadrature-phase (Q) components of carrier waveforms of thecurrent symbol. This results in each data value being associated with acorresponding complex number C_(i)=A_(i) exp(jΦ_(i)) whose real partcorresponds to the I component and whose imaginary part corresponds tothe Q component of a carrier with peak frequency f_(i). Alternatively,any appropriate modulation scheme that associates data values tomodulated carrier waveforms can be used.

The mapping module 222 also determines which of the carrier frequenciesf₁, . . . , f_(N) within the OFDM bandwidth are used by the system 200to transmit information. For example, some carriers that areexperiencing fades can be avoided, and no information is transmitted onthose carriers. Instead, the mapping module 222 uses coherent BPSKmodulated with a binary value from the Pseudo Noise (PN) sequence forthat carrier. For some carriers (e.g., a carrier i=10) that correspondto restricted bands (e.g., an amateur radio band) on a medium 204 thatmay radiate power no energy is transmitted on those carriers (e.g.,A₁₀=0). The mapping module 222 also determines the type of modulation tobe used on each of the carriers (or “tones”) according to a “tone map.”The tone map can be a default tone map, or a customized tone mapdetermined by the receiving station, as described in more detail below.

An inverse discrete Fourier transform (IDFT) module 224 performs themodulation of the resulting set of N complex numbers (some of which maybe zero for unused carriers) determined by the mapping module 222 onto Northogonal carrier waveforms having peak frequencies f₁, . . . , f_(N).The modulated carriers are combined by IDFT module 224 to form adiscrete time symbol waveform S(n) (for a sampling rate f_(R)), whichcan be written as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}\mspace{11mu} {A_{i}{\exp \lbrack {j( {{2\pi \; {in}\text{/}N} + \Phi_{i}} )} \rbrack}}}} & {{Eq}.(1)}\end{matrix}$

where the time index n goes from 1 to N, Ai is the amplitude and Φ_(i)is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R), andj=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 226 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 204. Thepost-processing module 226 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 200 and/or the communicationmedium 204) the post-processing module 226 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol. Thepost-processing module 226 can also perform other functions such asapplying a pulse shaping window to subsets of symbols within the symbolset (e.g., using a raised cosine window or other type of pulse shapingwindow) and overlapping the symbol subsets.

An Analog Front End (AFE) module 228 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 204. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 204 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 204 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 206, modules implementing the PHY layer receive a signalfrom the communication medium 204 and generate an MPDU for the MAClayer. An AFE module 230 operates in conjunction with an Automatic GainControl (AGC) module 232 and a time synchronization module 234 toprovide sampled signal data and timing information to a discrete Fouriertransform (DFT) module 236.

After removing the cyclic prefix, the receiver 206 feeds the sampleddiscrete-time symbols into DFT module 236 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 238 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including deinterleaving and descrambling).

Any of the modules of the communication system 200 including modules inthe transmitter 202 or receiver 206 can be implemented in hardware,software, or a combination of hardware and software.

Channel Estimation

Channel estimation is the process of measuring the characteristics ofthe power line channel to adapt the operation of the PHY layer toprovide optimal performance.

Channel estimation can include:

-   -   Selection of the tone map designating modulation method(s) to be        used on each carrier. Any given carrier may use different        modulations at different times within the beacon period.    -   Selection of the FEC rate.    -   Selection of the guard interval length.    -   Selection of the intervals within the beacon period where a        particular tone map, FEC rate, and guard interval setting        applies.

The FEC rate and guard interval length can vary over the AC line cycleperiod, but they are the same for all carriers at any given time.

The results of channel estimation can be reported to the CCo incentralized protocols for use in allocating time slots in the CFP. TheCCo can allocate time for communication between a transmitting andreceiving station to perform channel estimation. The CCo can then usethis channel estimation information in determining or updating theschedule of time slots allocated to stations in the CFP. In distributedprotocols, the results of channel estimation can be used by stations asthey coordinate communication including setting up time periods duringwhich unidirectional or bi-directional communication can occur between apair of stations.

The channel-estimation procedures may differ slightly between the CP andthe CFP. In the CP, the receiving station can designate a default tonemap that may be used by the transmitting station anywhere in the CP. Thereceiving station may optionally define additional Tone maps that may beused in the CP during particular intervals of the beacon period. Thisapproach allows the transmitting station to begin communicating usingtone map modulated data quickly, and avoids complicated interactionsbetween the CSMA access procedure and the channel-estimation proceduresfor the CP. This approach is well suited to the transport of best effortdata. Alternatively, the receiving station can designate intervalswithin a beacon period over which particular channel adaptation applieswithout taking into account whether a transmission is within the CP orthe CFP.

Before data communication occurs in the CFP, the receiving stationdefines a tone map that is valid in the interval of the beacon periodwhere the transmission is scheduled. If no valid tone map is defined inan interval, the transmitting station sends a “SOUND MPDU” in theinterval until the receiving station defines a tone map that for theinterval in a “Sounding process.” The SOUND MPDU includes a signal knownto the receiving station from which the receiving station can estimatecharacteristics of the channel.

An MPDU that carries data is called a “data MPDU.” To acknowledge thatthe PBs within a data MPDU have been successfully received the receivingstation sends to the transmitting station a selective acknowledgement(SACK) message. The SACK designates which of the PBs were successfullyreceived (e.g., using check sequence) and which of the PBs should beretransmitted. For a SOUND MPDU, a “Sound ACK” is used to indicate thereception status and completion of the Sounding process. In some cases,the SACK and Sound ACK messages can be sent as information within one ormore fields of the frame control of a short MPDU.

The receiving station defines a tone map in which the modulation for acarrier is tailored to the characteristics of the channel at thatcarrier frequency. In addition to channel characteristics, the receivingstation can also define a tone map based on a type of data to betransmitted (e.g., more robust modulation for applications moresensitive to data loss). The tone map is sent to the transmittingstation in a channel estimation response (CER) message.

Alternatively, if no valid tone map is defined in an interval, thetransmitting station can use a default tone map that has enoughredundancy to be successfully transmitted assuming worst case channelcharacteristics. This default tone map may be more appropriate if thetransmitting station only has a relatively small amount of data to send.

The channel-estimation procedures also include mechanisms formaintaining the lists of the intervals within the beacon period whereeach tone map may be used. Tone map intervals are defined as timeperiods within the beacon period where a particular tone map may beused. Since the CCo locks the beacon period to the AC line cycle,intervals are synchronized to the AC line cycle.

Channel and noise characteristics over the power line tend to beperiodic with the underlying AC line cycle. In some cases, theseimpairments occur at twice the frequency of the AC line cycle (i.e., 100or 120 Hz), while in other cases they may occur at the same frequency asthe AC line cycle (e.g., a noise source that responds to the polarity ofthe AC line waveform). Because of the different access mechanisms andQoS requirements, intervals occurring in the CP and CFP may be treateddifferently.

The receiving station specifies the intervals within which various tonemaps may be used, subject to certain guidelines which may include any ofthe following:

-   -   The CP default tone map may be used anywhere in the contention        period.    -   With the exception of default tone map, intervals are disjoint        (non-overlapping).    -   The transmitter may not transmit PPDUs with the PPDU Payload        crossing the boundary between intervals using different tone        maps.    -   The receiver specifies intervals that are large enough to carry        a complete PPDU, based on the indicated tone map.    -   The current intervals definition is carried in the CER message.    -   The current intervals definition becomes stale if a period of 30        seconds has elapsed since the last CER message was received from        the receiving station.

Bursting

Bursting is a process in which a station transmits multiple MPDUs in aburst (without relinquishing the medium) before soliciting a response(e.g., a SACK). For example, when a burst of data MPDUs is transmitted,the SACK transmitted at the end of the burst contains the receptionstatus of all the PBs in each of the MPDUs in the burst. When a burst ofSound MPDUs is transmitted, the Sound ACK that is transmitted at the endof the burst will indicate the reception status of the Sound MPDUs.Since bursts can be acknowledged by one response (after time spentwaiting for response and the subsequent interframe spacing) for a groupof MPDUs, they provide higher MAC efficiency. Significant improvement inthe performance can be obtained for high data rate streams, such as HighDefinition Television (HDTV) and Standard Definition Television (SDTV)streams, by using bursting. Receivers are configured to recognize burstsand wait to send acknowledgements until after the burst has ended.

FIG. 3 shows an example of bursting with three data MPDUs 300A, 300B,300C in a burst 302. The MPDUCnt field in the SOF delimiter of the firstMPDU 300A in the burst 302 indicates the number of MPDUs that follow thefirst MPDU in the burst. Following MPDUs include decremented MPDUCntvalues that indicate the remaining number of MPDUs. In this example, thefirst MPDU 300A, second MPDU 300B, and third MPDU 300C include MPDUCntvalues of 2, 1, and 0, respectively, in the SOF delimiters of the MPDUs.The burst of three data MPDUs ends with transmission of a SACK MPDU300D, which carries PB reception information for each of the MPDUs 300A,300B, 300C.

MPDUs are categorized as “regular MPDUs” or “burst MPDUs,” depending onwhether a response is expected at the end of their transmission. MPDUsthat are followed by a response are referred to as regular MPDUs and areindicated by setting the MPDUCnt in their delimiter to a predeterminedvalue (e.g., 0). The last MPDU in a burst and MPDUs in non-bursttransmissions belong to this category. MPDUs that are followed by one ormore long MPDUs are referred to as burst MPDUs. In this case, MPDUCntindicates the number of MPDUs to follow.

When a Data MPDU with a non-zero MPDUCnt field is received by adestination, the receiver refrains from generating a response and storesthe corresponding SACK information locally. This process continues untilthe last MPDU in the burst (indicated by MPDUCnt=0) is received. Uponthe reception of the last MPDU, the receiver aggregates the stored SACKinformation and transmits it in a single SACK MPDU. The SACK fieldscontain the reception status of each of the PBs in each of the MPDUs. Ifthe transmitter fails to receive a SACK at the end of an MPDU Burst, itmay send a SOF delimiter with Request SACK Retransmission (RSR) setto 1. This will cause the receiver to transmit (or retransmit) the SACKinformation.

When a Sound MPDU with a non-zero MPDUCnt field is received by thedestination, the receiver refrains from generating a Sound ACK and waitsfor the subsequent Sound MPDU. This process continues until the lastMPDU in the burst (indicated by MPDUCnt=0) is received. Upon receptionof the last MPDU, the receiver responds with a Sound ACK. In contrast toSACK, Sound ACK indicates the proper reception of the last Sound MPDU inthe Burst and does not necessarily carry an indication on the receptionstatus of other Sound MPDUs in the burst.

During the CP, the maximum duration of a MPDU Burst, including theresponse time and the subsequent Contention Interframe Spacing is shortenough to fit within the CP.

The burst MPDUs can include a predetermined minimum number of OFDMSymbols (e.g., at least two OFDM Symbols) for transmitting the MPDUPayload. This minimum number of OFDM Symbols can ensure that thereceiver has sufficient time to interpret the frame control and startsearching for the next MPDU in the burst.

In some implementations, the stations support a “bidirectional burst”procedure. This procedure allows a transmitting station to allocate partof its time to the receiving station, so the receiving station can senddata to the transmitting station (i.e., over the reverse channel). Thisprocedure can be used to send reverse traffic associated withapplications that require 2-way exchange of information betweenstations. For example, Transmission Control protocol (TCP) basedapplications have application data going in one direction and TCPAcknowledgments coming in the reverse direction. In such cases, thebidirectional bursting procedure can be used to send TCPacknowledgements in the reverse channel. Applications such as Voice overInternet Protocol (VoIP), Video conferencing, etc., can also takeadvantage of bidirectional bursting procedure. This procedure may alsobe used for exchange of network management information, for example, tosend an updated tone map to a transmitting station in response to achange in the channel characteristics as determined by the receivingstation. In general, the bidirectional busting procedure can be used inany scenario where there is traffic flowing between a pair of station inboth directions.

The receiving station initiates the bidirectional burst procedure usingfields in a SACK message transmitted during the receiving station'sallocated time interval for sending a SACK message. The fields in theframe control indicate the presence of the request (e.g., in a RequestReverse Transmission Flag (RRTF)) and the number of blocks that thereceiving station wants to send (e.g., in a Request Reverse TransmissionLength (RRTL) field). The receiving station can also indicate the typeof data to be sent.

Upon receiving the request, the transmitting station decides whether therequest will be granted and, in some implementations, the duration ofthe granted time. The transmitting station signals that the request isallowed by sending grant information including any relevant informationsuch as, for example, the maximum amount of data that may be transmittedin the reverse direction. The transmitting station can grant the requestby setting a Bidirectional Burst Flag (BBF) in the SOF delimiter, andcan indicate the duration in a Maximum Reverse Transmission Frame Length(MRTFL) field in the SOF delimiter.

In some implementations, the transmitting station does not need to sendthe duration of the time granted because the duration may bepredetermined before the request. For example, for example, the grantedtime may be a default amount of time (e.g., 1 ms). The receiver wouldsimply request a reverse transmission without needing to indicate anumber of blocks requested. The default amount of time can be pre-storedin each of the stations, or the default amount of time can be negotiatedwhen a station joins a network. In some cases the default amount of timecan be determined as a function of various parameters of one or morestations. For example, the default amount of time can be a function ofdata rate being used by a station. A higher data rate can be assigned asmaller amount of time since more data can be sent in a given timeperiod at a higher data rate. In some implementations, the transmittercan initiate granting a reverse transmission without receiving a requestfrom the receiver. For example, the transmitter may already know thatthe receiver will need time for reverse transmissions due tocharacteristics of a higher layer protocol.

FIG. 4 shows an exemplary timing diagram for the bidirectional burstprocedure. Transmissions in a “forward” direction from station A tostation B are shown on the top of a timeline 402, and transmissions in a“reverse” direction from station B to station A are shown in the bottomof the timeline 402. Initially, station A has been allocated time totransmit information to station B (e.g., after contending for access tothe medium). The bidirectional burst starts with a transmission fromstation A to station B. The transmissions from station A to station Binclude MPDUs with a SOF delimiter 404 and a payload 406. Initially,station B is allocated time to respond with a SACK transmission 408 foreach MPDU (or burst of MPDUs) but not time to send additionalinformation in the reverse direction back to station A. When station Bdetermines that it has information to send in the reverse direction, itsets the RRTF and RRTL fields in the SACK transmission. These fields areset until station A responds with a grant for reverse transmission(e.g., by setting the BBF flag to 1 and indicating the maximum durationof reverse transmission in the MRTFL field) or until there is no longera need to request a transmission in the reverse direction. Additionally,to reduce overhead, when station B is going to send an MPDU (or burst ofMPDUs) that includes a payload 406 in the reverse direction, station Bcan combine the SACK that would otherwise have been sent with the SOF ofthe transmission in a Reverse SOF (RSOF) 410 that includes the SACKinformation within the frame control of the RSOF 410.

In some implementations of the bidirectional burst procedure, there arerestrictions on the bursting of multiple data MPDUs before soliciting aSACK response. For example, in some implementations bursting can be usedin the forward direction, but not in the reverse direction. In thiscase, bidirectional communication is still allowed, but bursting onlyoccurs in the forward direction.

Sufficient interframe spaces are maintained between various MPDUsexchanged during bidirectional bursting to account for the processinglatencies and propagation latencies. FIG. 5 shows the various interframespaces for bidirectional bursting. A minimum interframe space of RIFS_1is present between the end of a forward MPDU and the start of subsequentreverse MPDU. A minimum interframe space of RIFS_2 is present betweenthe end of a reverse MPDU and the start of a subsequent forward MPDU. Aninterframe spacing of RIFS_3 is present between the end of a forwardMPDU and the start of the corresponding reverse SACK MPDU. The valuesfor RIFS_1, RIFS_2 and RIFS_3 can be fixed values or can vary based onimplementation. For example, the transmitter and receiver can negotiatethe values for these interframe spaces before the bidirectional burstingprocedure is used.

In some implementations, the bidirectional burst procedure depends onwhether the burst is occurring in a CP, or a CFP. Bidirectional burstsin a CP end with a SACK or other indication that the bidirectional burstis over. The SACK can be sent in either the “forward” or “reverse”direction (by either the original transmitter or original receiver,respectively), as described in more detail below. Bidirectional burstsin a CFP may end with transmission of a SACK (in either direction) ortransmission of a Reverse SOF and payload (the use of this option can becommunicated during connection setup). When a bidirectional burst in aCFP ends with a Reverse SOF and payload, a minimum interframe space ofRIFS_AV is present between the end of Reverse SOF payload and the end ofthe CFP. The support for CFP bidirectional bursts ending with ReverseSOF and payload enables stations to improve the efficiency of VoIP andother low data rate applications.

The unidirectional bursting and bidirectional bursting protocols can beused in a CFP or a CP of protocols that are centralized via a CCo orprotocols that are distributed without the need for a CCo. Incentralized protocols, the channel access is controlled by a singlestation (e.g., the CCo). The CCo provides dedicated channel accessintervals to various stations. The CCo may also provide sharedallocations to stations. During these shared allocations, multiplestations can transmit. For example, stations may use a distributedprotocol like CSMA during the shared allocation. Examples of centralizedprotocols include polling protocols and beacon based TDMA channel accessprotocols. Channel access in distributed protocols is managed bycoordination between various stations in the network, without the needfor a single station acting as a master. An example of distributedprotocols includes networks operating using contention based channelaccess protocol like CSMA as the basic channel access mechanism.Distributed protocols can also support contention free allocations. Forexample, station can negotiate contention free allocations with otherstation in the network, thus enabling the station to have access duringcontention free intervals.

Unidirectional or bidirectional bursting operating over distributedprotocols take into account the possibility of collisions. For example,a burst can be prematurely terminated when a collision is suspected(e.g., either directly detected, or inferred due to lack ofcommunication for a predetermined amount of time).

Unidirectional or bidirectional bursting over distributed protocolsprovide sufficient information to other stations in the network (i.e.,stations other than the two stations participating in the burst) toenable them to determine the end of the burst and when they can startcontending for access to the medium. In a technique used by some otherprotocols, the other stations would determine that they can startcontending for access to the medium by detecting a terminal inter-framespacing of a certain length. If no signals are detected for a givenamount of time, the stations would determine that they could startcontending. Such a terminal inter-frame spacing would need to be largerthan the inter-frame spacing used between transmissions in a burst. Inthe technique used in the unidirectional and bidirectional burstingprotocols described herein, the stations are able to detect the SACKtransmission (in either direction) and anticipate that they will be ableto start contending immediately (or within a short terminal inter-framespacing) after the end of the final transmission. Any unique indicatorwithin a final transmission (e.g., a SACK or a Reverse SOF and payload)by either the transmitter or receiver stations can signal that the burstis ending and that the other stations are able to start contending foraccess to the medium. In some cases, a terminal inter-frame spacing canbe much shorter than the inter-frame spacing within a burst due toprocessing latencies that may be required between transmissions in theburst.

Additional restrictions like providing an upper bound on the duration ofbidirectional bursting can be applied to ensure fairness among variousstations in the network.

FIG. 6A shows an example of bidirectional bursting during a CP (e.g., aduring a CSMA period within a beacon period, or during CSMAcommunication without a CCo station sending beacon transmissions). Whenstation A gains access to the channel (after contending with otherstations), it can set the BBF field in the SOF delimiter 600 to 1 toindicate that the channel will not be relinquished after the firsttransmission of a burst of frames from station A. Station B, indicatesto the other stations, by sending a Reverse SOF 602, that they cannotaccess the channel following that transmitted frame from station B.Station A can follow the receipt of RSOF 602 with the transmission ofanother frame or burst of frames. The transmission of a SACK (or otherpredetermined indicator) by either station A or station B is used toindicate the end of the bidirectional burst. In this example, station Aends the bidirectional burst with a SACK 604.

In another example shown in FIG. 6B, if after sending a reversetransmission station B continues to request a reverse transmission (asindicated by the RRTF and RRTL fields) and station A has no data tosend, station A may continue the burst by sending a short frame 610 witha SOF delimiter with the BBF field set to 1 but no payload. Similarly,if station A has granted time for a reverse transmission and station Bdoes not have any data to transmit, station B can continue the burst bysending a short frame with a RSOF delimiter but no payload. The sequenceof bidirectional bursts can be terminated with either station A orstation B transmitting a SACK. In this example, station B ends thebidirectional burst with a SACK 612.

In some examples, station A may also instruct station B to terminate thebidirectional burst by setting the BBF field to 0 in the SOF delimiter.Station B would then terminate the bidirectional burst by sending aSACK. If either station suspects a potential collision (e.g., if some orall received PBs are not correctly received), it can terminate thebidirectional burst with a SACK. Likewise, if no frame control isreceived after a predetermined amount of time, the sending station canassume a collision has occurred and terminate the bidirectional burstwith a SACK.

During a bidirectional burst, listening stations (i.e., stations notparticipating in the bidirectional burst) defer to the two stationsparticipating in the bidirectional burst until the end of the burst(e.g., indicated by a SACK). Upon receiving a SOF with MPDUCnt set to 0and BBF set to 0, the third-party stations would infer that thebi-directional burst is ending and they would start priority contentionat the end of the expected SACK transmission. If they receive an SOFwith MPDUCnt set to 0 and BBF set to 1, they would start looking for areverse transmission. If they receive a RSOF, they would continue tolook for a SOF. If they receive a SACK at any time, they are allowed tostart priority contention immediately after the SACK.

The bidirectional burst procedure during CSMA can be subject toadditional restrictions. For example, the total duration of thebidirectional burst, including the final SACK and subsequent ContentionInterframe Spacing (CIFS_AV), can be limited to not exceed apredetermined maximum time period (e.g., 5000 μsec). This maximum timeperiod may be determined to fit within a CSMA period within a beaconperiod, for example.

Many other implementations of the invention other than those describedabove are within the invention, which is defined by the followingclaims. For example, the bidirectional bursting can also be used inenvironments where the transmitter gets access to the medium using otherchannel access techniques.

What is claimed is:
 1. A method for communicating between stations in anetwork, the method comprising: determining that a first station hasbeen allocated a first time period to transmit over a shared medium inthe network; transmitting, from the first station to a second stationover the shared medium during the first time period, wherein stationsother than the first station and the second station refrain fromtransmitting over the shared medium during the first time period;receiving, from the second station by the first station, a requestmessage to allow the second station to transmit during the first timeperiod and a requested amount of time to transmit; determining, by thefirst station, whether to allow the second station to transmit duringthe first time period; and in response to allowing the second station totransmit during the first time period, determining, by the firststation, an authorized amount of time for the second station to transmitduring the first time period; and transmitting, by the first station tothe second station, an authorization message for the second station totransmit and the authorized amount of time.
 2. The method of claim 1,wherein the authorized amount of time for the second station to transmitduring the first time period is not less than the requested amount oftime.
 3. The method of claim 1, further comprising determining that thesecond station has been allocated a second time period that isnon-overlapping with the first time period.
 4. The method of claim 1,wherein the authorized amount of time during the first time period isdetermined as a function of a data rate being used by the secondstation.
 5. The method of claim 1, wherein in response to allowing thesecond station to transmit during the first time period, transmitting,by at least one of the first station and the second station, atermination notification during the first time period over the sharedmedium to cause termination of the first time period.
 6. The method ofclaim 5, further comprising coordinating among the stations to allocatea second time period to one of the stations to transmit over the sharedmedium after termination of the first time period.
 7. The method ofclaim 6, wherein transmitting the termination notification comprisestransmitting an acknowledgement confirmation successful transmission. 8.The method of claim 5, wherein transmitting the termination notificationis in response to detecting a potential collision.
 9. The method ofclaim 1, wherein the first station is allocated the first time period totransmit over the shared medium according to a carrier sense multipleaccess (CSMA) protocol.
 10. The method of claim 1, wherein determiningthat the first station has been allocated the first time period totransmit over the shared medium in the network comprises receivingallocation to the first station of the first time period to transmitover the shared medium in the network.
 11. A method for communicatingbetween stations in a network, the method comprising: determining that afirst station has been allocated a first time period to transmit over ashared medium in the network based on a contention period to determineselection among the stations; transmitting, from the first station to asecond station over the shared medium during the first time period,wherein stations other than the first station and the second stationrefrain from transmitting over the shared medium during the first timeperiod; in response to receiving, by the first station, a requestmessage from the second station that includes a requested amount of timethat is requested for transmission by the second station, transmitting,by the first station, an authorization message to the second station totransmit for an authorized amount of time during the first time period;and transmitting, by the first station, the authorized amount of time tothe second station.
 12. The method of claim 11, wherein the authorizedamount of time for the second station to transmit during the first timeperiod is not less than the requested amount of time.
 13. The method ofclaim 11, further comprising determining that the second station hasbeen allocated a second time period that is non-overlapping with thefirst time period.
 14. The method of claim 11, wherein the authorizedamount of time during the first time period is determined as a functionof a data rate being used by the second station.
 15. The method of claim11, wherein in response to allowing the second station to transmitduring the first time period, transmitting, by at least one of the firststation and the second station, a termination notification during thefirst time period over the shared medium to cause termination of thefirst time period.
 16. The method of claim 15, further comprisingcoordinating among the stations to allocate a second time period to oneof the stations to transmit over the shared medium after termination ofthe first time period.
 17. The method of claim 16, wherein transmittingthe termination notification comprises transmitting an acknowledgementconfirmation successful transmission.
 18. The method of claim 15,wherein transmitting the termination notification is in response todetecting a potential collision.
 19. The method of claim 11, wherein thefirst station is allocated the first time period to transmit over theshared medium according to a carrier sense multiple access (CSMA)protocol.
 20. The method of claim 11, wherein determining that the firststation has been allocated the first time period to transmit over theshared medium in the network comprises receiving allocation to the firststation of the first time period to transmit over the shared medium inthe network.
 21. A station in a network for communications, the stationcomprising: a network interface module configured to, determine that thestation has been allocated a first time period to transmit over a sharedmedium in the network; transmit to a different station over the sharedmedium during the first time period, wherein stations other than thestation and the different station refrain from transmitting over theshared medium during the first time period; receive, from the differentstation, a request message to allow the different station to transmitduring the first time period and a requested amount of time to transmit;determine whether to allow the second station to transmit during thefirst time period; and in response to allowing the second station totransmit during the first time period, determine an authorized amount oftime for the second station to transmit during the first time period;and transmit, to the second station, an authorization message for thesecond station to transmit and the authorized amount of time.
 22. Thestation of claim 21, wherein the authorized amount of time for thedifferent station to transmit during the first time period is not lessthan the requested amount of time.
 23. The station of claim 21, whereinthe network interface module is configured to determine that thedifferent station has been allocated a second time period that isnon-overlapping with the first time period.
 24. The station of claim 21,wherein the authorized amount of time during the first time period isdetermined as a function of a data rate being used by the differentstation.
 25. The station of claim 21, wherein the network interfacemodule is configured to transmit a termination notification during thefirst time period over the shared medium to cause termination of thefirst time period.
 26. The station of claim 25, wherein the terminationnotification comprises an acknowledgement confirmation successfultransmission.
 27. The station of claim 25, wherein the network interfacemodule is configured to transmit the termination notification inresponse to detection of a potential collision.
 28. The station of claim21, wherein the station is allocated the first time period to transmitover the shared medium according to a carrier sense multiple access(CSMA) protocol.
 29. The station of claim 21, wherein the networkinterface module configured to determine that the first station has beenallocated the first time period to transmit over the shared medium inthe network comprises the network interface module configured to receiveallocation to the station of the first time period to transmit over theshared medium in the network.
 30. A station in a network forcommunications, the station comprising: a network interface moduleconfigured to, determine that the station has been allocated a firsttime period to transmit over a shared medium in the network based on acontention period to determine selection among the station and otherstations in the network; transmit, to a different station of the otherstations over the shared medium during the first time period, whereinstations other than the station and the different station refrain fromtransmitting over the shared medium during the first time period; inresponse to receipt of a request message from the different station thatincludes a requested amount of time that is requested for transmissionby the different station, transmit an authorization message to thesecond station to transmit for an authorized amount of time during thefirst time period; and transmit the authorized amount of time to thesecond station.
 31. The station of claim 30, wherein the authorizedamount of time for the different station to transmit during the firsttime period is not less than the requested amount of time.
 32. Thestation of claim 30, wherein the network interface module is configuredto determine that the different station has been allocated a second timeperiod that is non-overlapping with the first time period.
 33. Thestation of claim 30, wherein the authorized amount of time during thefirst time period is determined as a function of a data rate being usedby the different station.
 34. The station of claim 30, wherein thenetwork interface module is configured to transmit a terminationnotification during the first time period over the shared medium tocause termination of the first time period.
 35. The station of claim 34,wherein a second time period is allocated to one of the other stationsto transmit over the shared medium after termination of the first timeperiod.
 36. The station of claim 34, wherein the network interfacemodule is configured to transmit the termination notification inresponse to detection of a potential collision.
 37. The station of claim34, wherein the station is allocated the first time period to transmitover the shared medium according to a carrier sense multiple access(CSMA) protocol.
 38. The station of claim 30, wherein the networkinterface module configured to determine that the first station has beenallocated the first time period to transmit over the shared medium inthe network comprises the network interface module configured to receiveallocation to the station of the first time period to transmit over theshared medium in the network.